RED PT500-2, Service Manual

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PT500-2 Service Manual
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Figure 5.7 Schematic Diagram for Audit Circuit of Receiver
The audio signal processing circuit of Transmitter consists of IC500 and the peripheral
components. Voice signals from MIC are sent to VCO for modulation together with
CTCSS/DCS after amplification, limitation and filtering.
AGC circuit consists of D500, D501 and Q503. The signal amplitude is reduced to ensure
no distortion in case of oversized MIC signal.
Q504, the power switch of voice processing circuit, controlled by MCU, will give power supply
to IC500 only during transmission.
J501 is the socket for external MIC, and the internal MIC will disconnect automatically when
using external MIC, but the internal PTT is still effective.
5.4 PLL Frequency Synthesizer
Figure 5.8 Schematic Diagram for Frequency Synthesizer
The PT500-2 adopts PLL frequency synthesizer.
Frequency synthesizer consists of reference oscillator, voltage controlled oscillator (VCO),
programmable frequency divider (PFD), phase comparator and low pass filter (LPF).
RX VCO Unit consists of Q4 D2 D4 L14 C35 C41 C44 C45 and D1 Q5.
TX VCO Unit consists of Q4 D2 D4 L14 C35 C41 C44 and C45,D5 is the modulation circuit
of VCO.
IC1 (MB15E03) is PLL integrated circuit and contains programmable parametric frequency
divider (PPFD), programmable frequency divider (PFD), phase comparator and charge
pump, etc.
Low pass filter consists of R5 R7 R9 R10 C12 C15 and C18.
Reference frequency is provided by X1 (TCXO, 12.8MHz).
Reference frequency of TCXO (Temperature-controlled Crystal Oscillator) is divided by
PPFD in IC1 to produce reference frequency of 5kHz or 6.25kHz (controlled by MCU based
on the set channel frequency).